Transport and retention of flounder larvae ( Platichthys flesus L.) in the Dollard nursery (Ems estuary)

Abstract

Variability in tidal transport of larval flounder ( Platichthys flesus L.) in the Dollard was investigated to estimate the quantity of larvae that are annually retained in the nursery based on simultaneous measurements of water transport and larval concentrations. Sampling was done three times per hour in the main channel in the Dollard, during 35 tidal cycles divided over 1993, 1994 and 1995. In 1993, additional sampling was carried out simultaneously during eight tidal cycles in a subsidiary channel. Larval concentrations showed marked tidal variations at all time scales and indicated a patchy spatial distribution. The estimated net tidal transport of larvae was not correlated to the net tidal transport of water. However, a higher net water transport coincided with higher retention, defined as the percentage of net larval transport divided by the flood larval transport. Also, relatively longer duration of the flood than the ebb period (tidal asymmetry) was positively correlated with both net tidal water transport and larval retention. The wind direction significantly affected the net water transport at the cross-section in the main channel, whereas wind speed was not significant. The flood transport, but not the ebb or net transport, was significantly higher during spring tide following full moon than during the other lunar phases. The net tidal transport of larvae was much more variable in 1994 than in 1993 and 1995, frequently resulting in ebb surpluses in larval transport. Despite higher mean concentrations, the estimated net larval input was lower in 1994 than in 1993. This corresponded neither with the observed abundance estimates of juvenile flounder in the Dollard in these two years, nor with the abundance index obtained from routine surveys. The total larval input better reflects the juvenile flounder abundance. Variability in water circulation, influenced by varying wind conditions, and its effect on larval transport and retention is an important source of variability in larval immigration in the nursery and is held responsible for a poor correlation between net larval input and juvenile abundance.